Field of the Invention
The present invention relates to the field of the transport and distribution of heat-sensitive products, such as pharmaceutical products and foodstuffs, and relates very particularly to technologies where the cold necessary for maintaining the temperature of the products is supplied by a cryogenic unit that operates in an open loop and implements:                direct injection of a cryogenic fluid into the transport case (very often liquid nitrogen);        
or                what is known as “indirect” injection of a cryogenic fluid into the transport case (very often liquid nitrogen), said “indirect” technique often being referred to as “CTI” and employing one or more heat exchanger(s) in the internal enclosure in which the products are transported (also known as “chamber”, “box”, isothermal “case”, etc.), the cryogenic fluid (such as liquid nitrogen or liquid CO2) flowing through said heat exchanger, the enclosure furthermore being provided with an air circulation system (fans) that brings this air into contact with the cold walls of the heat exchanger, thereby making it possible to cool the air inside the cold chamber of the truck, the cryogenic fluid feeding the heat exchanger(s) coming from a cryogen reservoir that is traditionally situated under the truck (the reservoir itself being fed, when this is necessary, from an upstream reservoir that is fixed or movable but in any case not attached to the vehicle).        
Throughout the following text, the term “reservoir” will denote the on-board cryogen reservoir, unless a specification such as “upstream” or “fixed” denotes a different reservoir.
The atmospheres maintained inside the cold chamber can be provided both for fresh produce (typically a temperature of around 4° C.) and for frozen foods (typically a temperature of around −20° C.).
The present invention relates more particularly to cryogenic solutions involving indirect injection but the solutions proposed can be applied very advantageously to cryogenic units involving direction injection of nitrogen, CO2 or any other cryogen.
Related Art
In the case of indirect injection, the heat removed from the air first of all allows complete evaporation of the cryogenic fluid flowing through the heat exchanger, then a rise in its temperature until it reaches a temperature close to that of the enclosure. The cryogenic fluid is then expelled to the outside after having transferred a maximum of cooling energy.
The method typically implemented in such trucks that operate with direct or indirect injection is most frequently controlled as follows:
1—while the refrigeration system of the truck is being started up (for example at the start of a round or after a lengthy shutdown of the refrigeration system for any reason) or after the opening of a door, a rapid temperature-drop mode is adopted (this phase is known as “pull-down” in this industry).
2—once the setpoint temperature has been reached in the product storage chamber, a control/regulation mode is adopted, making it possible to maintain the temperature in the product storage chamber at the setpoint value (“holding”).
However, the refrigeration needs in each of these two phases, in terms of refrigeration power required, are extremely different.
Specifically, in the “pull-down” phase, there is often a demand for the temperature of the air in the chamber to drop rapidly. In order to obtain this effect, it is necessary to provide high refrigeration power that is capable of overcoming the thermal inertia of the entire system (air, cryogenic unit, walls of the truck) and the inlet of heat through the walls of the truck and via the opening of its doors. These refrigeration needs drop dramatically in the holding phase, given that only the inlet of heat through the walls continues.
In other words, the refrigeration needs of a truck during a given round fluctuate between two levels which can be referred to as “full load” and “partial load”, as the appended FIG. 1 clearly shows.
While the refrigeration power during the holding phase absolutely has to reach a required minimum level, that corresponding to the full load remains at the discretion of the designer of the refrigeration system within the standards applied in this field (ATP, DIN, etc.) that recommend a power of the installed refrigeration unit of at least equal to 1.75 times the power at partial load, this power mainly being dictated by the heat input through the walls (KSΔT). Clearly, the greater the full load power, the more a drop and rapid return of the air temperature inside the chamber to the setpoint temperature can be ensured.
Existing cryogenic systems operate for example with “nominal” pressure in the reservoir at a virtually fixed level of around 3.2 barg. Operational modularity is currently most frequently obtained through regulation of the valves for injecting the liquid in the all or nothing (“AON”) mode or in the proportional mode.
The appended FIG. 2 illustrates the schematic diagram of pressure regulation of the reservoir as commonly carried out at present in this field.
It shows what a person skilled in art is familiar with: the feed path EV LIN CTI for liquid to the heat exchanger(s) inside the chamber of the truck, and a path known as “RMP” (“rapid pressurization”) for repressurizing the atmosphere of the reservoir.
This operation has a number of drawbacks:                1—if the pressure is lower than the required level during the filling of the reservoir (this occurring frequently in practice), the power that the cryogenic unit is supposed to produce decreases rapidly. This results in a fairly long “pull-down” time and overconsumption that is harmful to the economic balance of the system. The appended FIG. 3 illustrates these phenomena, giving results of experiments that show this variation in pressure on account of the variation in the pressure in the reservoir;        2—since operation takes place at a fixed reservoir pressure, the extent of the modularity (the difference between the full load power and partial load power) remains limited with overconsumption of the cryogen on account of the effects of thermal inertia in the system. In other words, it is very difficult to achieve a “boost” mode in which a very high refrigeration power is sought;        3—the nominal pressure in the reservoir as is conventionally used today (for example 3.2 barg) lengthens the filling time from the upstream fixed large store (source), which is generally kept at around 4 barg. This results in a loss of cryogen during filling in the form of gas (gas flash) since the difference in pressure between the fixed and movable reservoirs remains low, hence a long filling time (typically 10 to 15 min).        